1. Field of the Invention
The present invention relates to an electron gun for a color cathode ray tube. More particularly the invention relates to an electron gun for a color cathode ray tube provided with a mechanism for deflecting the electron beams from cathodes to make the beams cross at the downstream side of a quadrupole lens and a cathode ray tube using the same.
2. Description of the Related Art
FIG. 4 is a cross-sectional view for explaining the internal structure of a Trinitron electron gun 1, while FIG. 5 is an enlarged view of the portion of FIG. 4 surrounded by the broken line. Note that, a half side to the left of the center axis is shown in FIG. 5.
As shown in FIG. 4 and FIG. 5, the Trinitron gun 1 is provided with cathodes 2 comprising a red cathode 2R and a blue cathode 2B arranged in-line around a green cathode 2G. The cathodes 2R, 2G, and 2B respectively emit a red, green, and blue electron beam.
From the cathodes 2 to the downstream emission of the electron the beams successively encounter a G1 electrode, G2 electrode, GMA electrode, GMB electrode, G3 electrode, G4 electrode, convergence electrode CONV, aperture grille (not shown), and three-color phosphor screen stripes (not shown). The electrodes are each provided with three openings (beam through holes) through which the red, blue, and green electron beams pass. For example, as shown in FIG. 5 the GMA electrode and the GMB electrode are each provided with beam through holes 9A and 9B through which the red electron beam passes, beam through holes 10A and 10B through which the green electron beam passes, and the beam through holes (not shown) through which the blue electron beam passes.
Here, the G1 electrode is for example supplied with a voltage of 0V. To simplify the structure, it is formed integrally with the cathodes 2. Further, the cathodes 2 are supplied with a voltage of 40 to 170V.
The G2 electrode, GNA electrode, GMB electrode, and G3 electrode cooperate to form a focus lens which bends the electron beams slightly inward around the green electron beam. Note that the G2 electrode is supplied with a voltage of 500V, the GMA electrode is supplied with a voltage of 7000V, the GMB electrode is supplied with a voltage of 7000V, and the G3 electrode is supplied with a voltage of 29000V.
Here, the voltage applied to either the GMA electrode and the GMB electrode is fixed, while the voltage applied to the other is variable.
Note that the distance L1 between the emission surface of the cathode 2G and the G1 electrode is about 0.1 mm, the distance L2 between the G1 electrode and the G2 electrode is about 0.4 mm, the distance L3 between the G2 electrode and the GMA electrode is about 1 mm, the distance L4 between the center of the GMA electrode and the center of the GMB electrode is about 0.5 mm, and the distance L5 between the GMB electrode and the left side end of the G3 electrode in the figure is about 2.0 mm.
The GMA electrode and the GMB electrode form a quadrupole lens for correcting the aspect ratio of the cross-section of the electron beam spot In order to obtain a good electron beam spot over the entire region of the phosphor s creen, that is, a good focus characteristic.
The G3 electrode, G4 electrode, and G5 electrode form a larg e diameter main focus lens (main lens) about which the beams cross. The red and blue electron beams cross, then spread out toward the outside and are deflected by the convergence electrode CONV to pass through the aperture grille and converge at the three-color phosphor screen.
In this way, the Trinitron electron gun 1 is configured with a single main lens for the three electron beams.
In the conventional Trinitron electron gun 1, however, with only the prefocus lens composed of the G2 electrode, GMA electrode, GMB electrode, and G3 electrode, it is not possible to sufficiently bend the electron beams so as to cross the beams in the main focus lens, so as shown in FIG. 1 and FIG. 2, the cathodes 2R and 2B are provided inclined by a predetermined angle toward the center axis Z.
Alternatively, instead of inclining the cathodes 2R and 2B, it is possible to arrange them in parallel with the cathode 2G (center axis Z) and use the voltage occurring at a cup-shaped wall surface provided in the vicinity of the outer circumference of the G2 electrode to bend the red and blue electron beams toward the center axis Z.
In the above Trinitron electron gun 1, however, If the cathodes 2R and 2B are arranged at an angle, the red and blue electron beams will strike the quadrupole lens composed the GMA electrode and the GMB electrode at an angle in accordance with that, so there will be the problem that the clearance between the beam through holes provided at the GMA electrode and GMB electrode and the beam paths will become small and, due to optical aberration, a sufficient focus characteristic will not be obtained. Further, if the clearance between the beam through holes and the beam paths is small in this way, there will be the problem of a low freedom of design. Further, it will be necessary to use a sophisticated and expensive manufacturing apparatus to arrange the cathodes 2R and 2B inclined precisely by a predetermined angle with respect to the center axis Z in the manufacturing process-resulting in the problem of a high cost.
Further, in the method of providing a cup-shaped wall surface in the vicinity of the outer circumference of the G2 electrode, in addition to the problem of the inability to obtain a sufficient focus characteristic, due to the effect of the medium voltage electrodes provided between the G2 electrode and the G3 electrode, that is, the GNA electrode and the GMB electrode, there will be the problem of a difficulty in obtaining a sufficient potential difference for bending the electron beams to cross them.